1. Field of the Invention
The present invention relates to a low-capacitance bidirectional device of protection against overvoltages. More specifically, the present invention relates to a bidirectional protection device intended for high-frequency applications.
2. Discussion of the Related Art
FIG. 1 shows a conventional structure of a bidirectional protection device 10. As usual in the representation of integrated circuits, the different regions are not drawn to scale.
Device 10 is formed of a monolithic circuit in which are formed two vertical Shockley diodes in antiparallel. The first vertical Shockley diode 11 is shown to the left of FIG. 1, and the second vertical Shockley diode 12 is shown to the right of FIG. 1.
The monolithic circuit includes a lightly-doped substrate 15 of conductivity type N. Substrate 15 includes a heavily-doped well 16 of conductivity type P on the side of its upper surface 17, and a heavily-doped well 18 of conductivity type P on the side of its lower surface 19.
First Shockley diode 11, located to the left of FIG. 1, includes an intermediary buried region 20, of conductivity type N, more heavily doped than substrate 15 but much more lightly doped than P-type well 16, arranged at the interface between P-type well 16 and substrate 15, and a heavily-doped N-type cathode region 21, arranged on the side of upper surface 17 of substrate 15 in P-type well 16. Cathode region 21 consists, for example, of spaced apart concentric rings, in parallel strips, or in islands separated according to a network.
Similarly, second Shockley diode 12, located to the right of FIG. 1, includes a buried region 22 of conductivity type N, and an N-type heavily-doped cathode region 23.
Upper and lower insulating regions 26 and 27 respectively cover the periphery of upper and lower surfaces 17 and 19 of substrate 15.
Upper and lower metallization layers 30 and 31 respectively cover the upper and lower surfaces 17 and 19 of substrate 15. Upper metallization layer 30 acts as the cathode electrode of first Shockley diode 11 by being connected to N-type cathode region 21, and of the anode electrode of second Shockley diode 12 by being connected to P-type well 16. Lower metallization layer 31 acts as the role of the anode electrode of first Shockley diode 11 by being connected to P-type well 18, and of the cathode electrode of second Shockley diode 12 by being connected to N-type cathode region 23. Upper and lower metallization layers 30 and 31, respectively, are connected to terminals A and B of device 10.
Conventionally, when the voltage applied across bidirectional protection device 10 is included between positive and negative break-over voltages, device 10 is non-conductive, and said to be in the off state. When the voltage is greater than the positive break-over voltage or smaller than the negative break-over voltage, the device is conductive. For the device to switch from the on state to the off state, the current flowing therethrough must fall below a threshold level.
For the two break-over voltages to have substantially the same absolute value, the dopant concentrations respectively of buried regions 20 and 22, and of P-type wells 16 and 18, must be identical.
In the off state, device 10 of FIG. 1 exhibits a general capacitance that may be high, which is a disadvantage upon use of protection device 10 for high-frequency applications, for example, applications in telecommunications.
A possibility, to decrease the general capacitance of device 10 in the off state, is to replace a device 10 having the desired break-over voltage by two identical sub-devices 10 assembled in series, each having a break-over voltage equal to half the desired total break-over voltage. The capacitances of the elementary sub-devices being assembled in series, the total capacitance is equal to the capacitance of one sub-device divided by two. However, when the break-over voltage of the bidirectional device of FIG. 1, which can be obtained, for example, by increasing the dopant concentration of buried regions 20, 22 (the dopant concentrations of wells 16, 18 remaining constant) is divided by two, its capacitance appears to increase. Thus, the decrease in capacitance obtained by the series assembly is less than expected.